In its elementary form, a printed circuit board includes, as a component, a dielectric layer of an epoxy resin-impregnated woven glass fiber body which is known as a "pre-preg." On one or both sides of the prepreg is bonded a conductive copper foil sheet. Subsequently, much of the copper, through a number of processes, including photographic, is etched away to leave conductive paths on the surface of the prepreg layer. When so assembled, the lamination is often called a core or a board. It is not uncommon to assemble a stack of such boards. The assembly is called a press lay-up and the stack is called a book. An entire press lay-up is heated and subjected to pressure. Upon curing and cooling, the then bonded individual boards are separated from each other and subjected to further processing in well known manners.
Of utmost importance in the manufacturing process is the maintenance of cleanliness or lack of contamination of the copper foil sheets. This is true whether or not the printed ciruit board has one copper layer on one side of a prepreg or is a compound board of numerous layers. A clean environment is one way to avoid, or at least minimize, contamination problems from resin dust, fiberglass fibers, hairs, bugs, and the like, which could cause defects in the copper circuit of the finished printed circuit board.
Another way is to use a protective film as disclosed in U.S. Pat. No. 5,120,590 in which the marginal edges of the protective film are removably joined to a copper foil layer. During use, the entire marginal edge area is cut off and thrown away, creating a substantial waste problem. Alternatively, if the protective film and copper foil are only joined at an end, there is substantial problem with maintaining alignment between the two, resulting in a contaminated copper foil.
Alternatively, U.S. Pat. No. 5,153,050 discloses the use of an aluminum sheet adhered along its marginal edges to two copper foil layers. This system is for use between two epoxy resin-impregnated woven glass fiber bodies. The aluminum sheet is generally thrown away leading to increased waste disposal. There have been substantial problems with panel alignment both before and during pressing as well as image transfer problems.
Other attempts at solving the same problems, while avoiding some of the drawbacks of the above systems, have failed to be commercially viable, apparently due to inoperativeness. For example, U.S. Pat. No. 5,057,372 discloses the use of various polyolefin based adhesives to coat the entire surface of the protective film layer. PCT/US95/08436 (WO96/01605) and PCT/IB96/01151 (WO97/15446) disclose the use of electron-beam curable adhesives particularly saturated copolyesters with terminal acrylic double bonds.
Accordingly, there remains a need for a system which will provide improved protection to minimize contamination problems of copper foils in the printed circuit board industry while simultaneously overcoming the problems noted above.